Texas report highlights water benefits of coal-to-gas switching

Aside from emitting less carbon dioxide (CO2), switching much of the Texas power generation portfolio from coal-to-natural gas could also conserve a lot of water.

That’s the crux of a newly-released report from researchers at the University of Texas at Austin.

“Replacing Texas’ coal-fired power plants with natural gas combined cycle plants (NGCCs) would reduce annual freshwater consumption in the state by an estimated 53 billion gallons per year, or 60% of Texas coal power’s water footprint, largely due to the higher efficiency of NGCCs,” according to the report.

That’s enough water conserved to supply a million people, assuming 140 gallons of daily consumption per person, according to the report.

Study looks at plant cooling water use and fuel extraction issuesThe study looks at the water intensity of fuel extraction for Texas lignite coal, conventional natural gas and 11 unconventional natural gas basins in Texas. The study also touched upon “second-order impacts associated with multi-stage hydraulic fracturing” or fracking.

“Despite the rise of this water-intensive natural gas extraction method, natural gas extraction, appears to consume less freshwater than coal per unit of energy extracted in Texas because of the high water intensity of Texas lignite extraction,” according to the report.

Natural gas has been grabbing a larger ratio of new power generation capacity largely because of its falling price and smaller emissions footprint compared to coal, the report notes.

The relative water intensity of coal and natural gas fuel cycles has received less attention, the report authors say. Texas produces all of its natural gas and about a third of its coal energy, primarily lignite, according to the report. As a result, Texas is a net importer of coal.

Water use for energy is an increasingly big deal in Texas. Virtually all of Texas experienced drought at some point in 2011 and dry conditions left over 11 GW (about 1%) of power generation capacity at risk of curtailment due to water limitations. Securing water rights has become a major challenge for new power plants, according to the plant.

This limited life cycle analysis assesses freshwater consumption associated with fuel extraction, power plant cooling and pollution controls, which together typically account for over 95% of electricity-related freshwater consumption, according to the study.

Included in the evaluation of water needs for pollution controls are carbon capture and storage (CCS) systems, which are not yet operating in Texas but could be installed in response to future CO2 control policies.

The study says about 60% of Texas’ coal-fired generation uses once through cooling, with the remaining 40% using wet recirculation. Texas’ NGCC generation is overwhelmingly cooled with wet recirculating systems (over 90%), with the remainder using once through (about 5%) or air cooling.

Texas coal-fired power generation accounted for an estimated 90 billion gallons of freshwater consumption in 2007 from its full fuel cycle, including about 10 billion gallons from mine dewatering. Generating the same amount of power from Texas fleet-average NGCC units would consume 37 billion gallons of Texas freshwater, according to the report.

The study found that Texas coal extraction consumes over seven times as much freshwater per KWH as Texas natural gas extraction. This finding is somewhat unexpected given the amount of water used for hydraulic fracturing, the report authors said.

When consumption from dewatering coal mines is excluded, Texas coal and Texas natural gas extraction consume roughly the same amount of freshwater per kWh. One major reason for the similarity is that existing NGCCs are about 30% more efficient than pulverized coal units and thus require less fuel to produce electricity.

This research was funded by the George and Cynthia Mitchell Foundation, the U.S. Department of Energy and the U.S. National Science Foundation’s EFRI-RESIN (Emerging Frontiers in Research and Innovation for Resilient Infrastructures) program.

Wayne Barber is chief of power generation for Energy Central’s Generation Hub, where this column first appeared.

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